collider physics hauser
collider physics hauser
Some slides copied from Peter
Richardson (Durham U.)
A short physics introduction
Why Hadron Colliders?
Going beyond fixed-target and e+e- circular colliders.
Constituents of the proton.
How to calculate cross-sections in proton collisions.
Current generation of hadron colliders
Electroweak unification and the Higgs Boson
Tevatron at Fermilab near Chicago.
Large Hadron Collider at CERN (Switzerland)
Future linear e+e- and muon colliders
Ideas of Force Unification
Weak + Electromagnetic
• Energy scale about 100 GeV.
• The theory hinges on the “Higgs” particle, energy<1000 GeV.
• Enigmatic Higgs particle is not yet observed, does it exist?
• If the Higgs doesn’t exist, there is a theorem that there
must be some additional force to be discovered, with
effects visible in the <1000 GeV range.
• Goal of the “current” generation of colliders is to find the
Higgs or its replacement.
• Current “Tevatron” probably can’t find the Higgs, future “LHC”
probably will (year ~2009).
Why Hadron Colliders (I)?
Before colliders, there was “fixed-target” –
a beam of particles hits a block of matter.
– Modern era of accelerators started in 1931.
– Relativistic disadvantage: ECM increases slowly
Colliders: counter-rotating beams within a
– Advantage: ECM = 2*Ebeam increases faster
– Developed around 1970.
– Need pretty intense beams for a worthwhile
rate of interactions.
Accelerators: Bigger, and Stronger
Magnets Higher Energy
1931 Lawrence and
the first Cyclotron
This is still
Modern Particle Accelerators
The particles are guided
around a ring by strong
magnets so they can
gain energy over many
cycles and then remain
stored for days
The particles gain energy by
surfing on the electric fields of
well-timed radio oscillations (in
a cavity like a microwave oven)
Historical Development of Colliders
Beam hits matter
Why Hadron Colliders (II)?
Electron-positron (e+e-) colliders (>1970)
– The Feynman diagrams are simple. e- e+
– Positrons circulate in the same set of magnets
as the electrons.
– You know the initial state energy and
momentum (zero) precisely by the magnetic
field and the radius of the electron path.
Many great things were discovered with
Why Hadron Colliders (III)?
Electrons are by far the lightest particles.
Therefore, magnetic fields accelerate them a lot, and acceleration of
charges results in electromagnetic “synchrotron” radiation.
For highly relativistic particles, this radiation depends on the
relativistic g=E/mc2 factor as:
So electrons, having the lowest mass, radiate like crazy if E (hence
g) is high.
Circular e+e- machines topped out with the LEP accelerator, which
reached about 200 GeV energy, which required massive amounts of
power to keep the electrons going around.
So if you want more center-of-mass energy than that, use protons,
which are 2000 times more massive than the electron.
Colliding Proton/Antiproton Beams
No problem with synchrotron radiation energy loss, but…
Like throwing bags of marbles at each other at high velocity:
Marble-marble collisions are interesting, not bag-bag collisions
Fortunately, the number and arrangements of the “marbles” has
been measured by other experiments
Proton Constituent Particles
• Proton internal structure is quarks and gluons
• Also, there are 2 up (u) and 1 down (d) “valence” quarks
• There are also gluons, holding them together, that carry 50%
of the proton momentum!
• There are also “sea” quarks!
• Argh – at a fundamental level, what is the beam??
• Suppose the beam energy was 1000 GeV, and you knew that
each valence quark carried 1/6 of the proton momentum, and
there were 5 gluons carrying 1/10 of the proton momentum,
and there were no sea quarks.
• Then you would have 2 u and 1 d quarks of 167 GeV, and 5
gluons of 100 GeV in each proton.
• Assuming no multiple scattering (Born approximation of
scattering Quantum Mechanics), you could calculate all the
Real Hadron Scattering
• The constituents don’t take on exact fractions of proton
momentum, but have probability functions called “parton
distribution functions” (PDFs)
• PDFs measured in high-energy electron-proton and neutrinoproton experiments. Good cross-checks between them, so it
all makes sense.
• The initial state is a probability function you have to
• For example, for quark-antiquark scattering:
s = Integral [ (fundamental cross-section) * (prob that beam A has quark
with momentum PA) * (prob that beam B has antiquark with momentum
Let’s start by thinking about how the top
quark is produced
The top quark is produced by the strong
• A tracking chamber
measures the energies of
charged particles (with aid
of a big magnet to bend
• A calorimeter measures
energies of neutral particles
• A muon system sees only
penetrating muon particles
• Used to take pictures
(bubble chambers), now we
use fully electronic readout
Timeline of Proton Collider
W/Z bosons Top quark
at Fermilab (Chicago)
The CDF (Collider
Detector at Fermilab)
accelerator, 6 km
The LHC (Large Hadron Collider) at
the CERN Laboratory
The LHC is built in the old LEP
tunnel at CERN near Geneva.
It will collide protons at an
energy of 14 TeV starting in
There will be four experiments.
Two ATLAS and CMS to look
for the Higgs, LHCB to look at
B physics and ALICE for heavy
Here I will concentrate on the
physics for ATLAS and CMS.
The two general
and CMS follow the
general design we
have just considered.
The CMS Endcap Muon System
• Chambers produced
• Equipping with
testing at UCLA
• 300,000 data
electronics built by
• Support from UC
Riverside and UC
40 million events/sec
x10 million sec/year (30% run eff.)
Search for Higgs particle
Look for data > background rate
~40 events excess
Each Higgs event is like a 1g needle in a 100
million metric ton haystack
How to Find Needles in Large
I) Special-purpose 40 MHz
“Level 1 Trigger”
II) Fast “online” Computers
“Level 2 Trigger”
UCSD & UCLA
III) “Offline” Analysis
Crunch Petabyte data store
(1 Million Gigabytes)
By looking at a large
number of different
signals the LHC can
discover the Higgs over a
wide mass range.
This range more than
covers the mass excepted
from the precision
The LHC should discover
Hadron vs. Lepton Collisions
The Tevatron is currently running at 1.96TeV centre-ofmass energy colliding protons and antiprotons. Will
continue till LHC start up in 2007.
The LHC will start in 2007 and run for about 10 years
colliding protons at 14 TeV.
People are working hard on the design of electronpositron Linear Colliders for the future.
– No synchrotron radiation
– But have to accelerate the particles very forcefully to reach high
energy in one pass (linear accelerator already many km long)
The Long-Term Future of Colliders
A future linear collider (ILC for International Linear
Collider) operating between 500 GeV and 1 TeV will
hopefully be built to start data taking some time after
A second generation linear collider operating at energies
of up to 5 TeV could then be built to explore higher
energies (e.g. the CLIC research project at CERN).
These machines will measure the properties in more
• There are other proposals to build a neutrino factory as
the first step towards a muon collider. This would be on
the same sort of timescale as a second generation linear